Clothing manufacturers are concerned about the danger that needles used for sewing clothes, bags, pouches, etc., if they are left to remain in the products, could prick the human body and to prevent such danger, magnetic inspection for detecting needles has been conducted. Accordingly, nonmagnetic plating, for example, nickel-phosphorus plating or nickel-tin alloy plating has been predominantly used for ornamental articles for clothing. However, in recent years, it has been pointed out that if nickel-containing metal contacts a human body, it can be an allergen to cause skin rashes or inflammation. Several countries in the world, such as European countries and the United States of America, are going to take some measures (legislation) for protecting the human body from such a nickel allergy.
Under the circumstances, copper-tin alloy plating has been reviewed in recent years as promising metal plating that substitutes for nickel alloy plating.
Many techniques have hitherto been proposed for copper-tin alloy plating as disclosed in JP 10-102278 A, JP 2001-295092 A (U.S. Pat. No. 6,416,571), JP 07-246562, and others. However, the conventional techniques have a problem of instability of a disengaging force. That is, when the conventional technique is applied to ornamental articles like snap buttons, which are attached to clothes and repeatedly subject to resilient snap engagement, fluctuation of the disengaging force (i.e. force required for disengaging a snap engagement) becomes greater as engagement and disengagement are repeated, and eventually the disengaging force will be outside of a specific range. As a result, when the disengaging force is too strong, the cloth will be ruptured and on the contrary when the disengaging force is too weak, the button will be disengaged of itself. Note that as shown in FIG. 1, which is a cross-sectional view showing a snap button, the snap button includes snaps used as a set, i.e., a male snap 1 consisting of a stud member 3 having a round head 3a that has a generally extended (flared) top and a fitting member 4 for fitting the stud member 3 to a cloth 7, and a female snap 2 having a socket member 5 resiliently engageable with the round head 3a of the male snap 1 and a fitting member 6 for fitting the socket member 5 to another cloth 8.
Further, when copper-tin alloy plating is applied to clothing ornaments, accessories or the like, the appearance color (color tone) of the plating is considered to be one of the important qualities required. In the copper-tin alloy plating, those platings having a red, yellow (gold), white, or silver white color tone have been realized by varying the contents of copper and of tin in the plating and on the other hand those platings having a black-based color tone have been realized by incorporating cobalt or selenium in the copper-tin plating.
However, since the use amounts of cobalt and selenium in the copper-tin-(cobalt or selenium) alloy in the plating having a black-based color tone are regulated by European Toy Safety Standard EN71-3 or Ecotex Standard 100, copper-tin alloy plating having a black-based color tone without containing any such controlled substances has been demanded.
As far as is known, there has been made only one proposal for the copper-tin alloy plating having a black-based color tone containing no such prohibited substance. That is, JP 10-102278 A discloses a method of producing copper-tin alloy plating having a pale black color tone with a Cu/Sn weight ratio=41/59. The black plating taught in the document has poor disengaging force stability and poor adhesion. As a result, a problem arises, for example, that the plating migrates to the clothes by friction with the clothes, so that the commercial value of the clothes is deteriorated, which prevents commercialization of the above-mentioned pale black copper-tin alloy plating.
Further, industrially operative plating having a black-based color tone for ornamentation and corrosion resistance includes nickel-tin alloy. However, the plating has poor adhesion so that its disengaging force stability is poor and, further, it causes a problem of nickel allergy.
In JP 07-246562 (Hoshi et al.), the alloy described therein is a sintered alloy, which is different from the plating of the present invention, which is a non-sintered allow.
As a production method of a metal bonded grinding wheel, the Hoshi reference describes in claim 6 a method including the following steps:                (i) a step if preparing plated abrasive grain 10 by forming metal plating layer 3 on the surface of super abrasive grain 2 through non-electrolytic plating method (FIG. 3(a) to (b));        (ii) a step of covering the grain 10 with particles through a pressure-bonding process, wherein the plated grain 10 is mixed in a mixture of particles consisting of Cu and Sn particles both having smaller average particle size than the plated grain 10 and the metal particle mixture is pressure-bonded on the metal plating of grain 10 through mechanical friction-pressure welding action in pressure-rolling motion in the presence of oxygen to form pressure-bonded covering layer 11 on the outer periphery of grain 10 and thereby obtain metal-coated grain 12, and wherein during the process the mixture of the particles is allowed to contain oxygen, (FIG. 3(b) to (c) and FIG. 4), and        (iii) a molding step, wherein the metal-coated grain 12 is subjected to pressure-molding and sintering or to hot-pressing and thereby pressure-bonded covering layers 11 are bonded to each other (FIGS. 1 and 2), to prepare sintered alloy containing 20 to 90 wt. % of Cu, 5 to 50 wt % of Sn and 0.5 to 3 wt % of oxygen and form metal-bonded abrasive grain layer 1 from the alloy.        
That is, “alloy including 20-95 wt % Cu, 5-50 wt % Sn and 0.5-3 wt % oxygen on super abrasive grains” corresponds to metal-bonded abrasive grain layer 1, which comprises sintered alloy prepared by the above step (ii) involving preparation of metal-coated grain 12 having pressure-bonded covering layer 11 thereon and then sintering of the grain through pressure-molding and sintering ort hot-pressing. Therefore, the alloy of Hoshi is poor in compositional uniformity, in that pores 6 are formed among particles of metal-coated grain 12 as shown in FIG. 2 and that even without pores, interface 5 is formed among the particles as shown in FIG. 1. Moreover, the thus formed portion like a spherical shell, obtained through pressure-bonding of particles followed by sintering, is poor in compositional uniformity at the micro level, and also properties of the portion differ depending on the particle size and particle size distribution of the powder used.
In contrast, the present invention relates to a plating alloy, which has uniformity at almost the molecular level, is different from aggregates of powder such as a sintered alloy.
Furthermore, it is well known that properties of alloys, even with the same composition, widely vary depending on the production methods. Accordingly, it can be easily understood by one of ordinary skill in the art that a plating alloy is different in structure and properties from a sintered alloy using powder metallurgy.
For example, if the same materials are used and the film thickness values are the same, a plating alloy is superior to a sintered alloy in terms of quality, such as corrosion resistance. Therefore, the non-sintered plating alloy of the invention is different from the sintered alloy of Hoshi in structure and properties. Further, intended uses of the two are completely different. The plating adopted in Hoshi is also clearly different from the Cu—Sn—O plating of the present invention.